T4 phage mutants with effects on genetic recombination are being examined for allelism and double mutants for their phenotypes in efforts to place events into metabolic pathways. Their proficiencies in repair of chemical lesions on T4 DNA are being examined, and one possibly important nuclease activity is under extensive study. The molecular state of phage P1-mediated transducing fragments is under investigation both in recombination-proficient and in rec- recipient bacteria in efforts that should outline both normal recombination mechanisms and, also, the exact points during molecular interaction at which E.coli rec-mutations are expressed.